1. Field of the Invention
The present invention relates generally to an image pickup system having a solid state image sensor device.
2. Description of Related Art
In recent years, MOS solid stage image pickup elements are widely noticed since there are advantages in that required voltages and costs are low and only one power supply is required.
Since a signal processing circuit for processing signals, which are outputted from a solid stage image sensor device having MOS solid state image pickup elements, comprises MOS transistors, the solid state image sensor device can be fabricated in the same fabricating process as a process for fabricating the signal processing circuit and can be formed on the same substrate as the signal processing circuit. For example, as shown in FIG. 15, as an example of an image pickup system having a MOS solid state image sensor device, a video camera 60 comprises: a MOS sensor 61, which is an image pickup element; an automatic gain control system (which will be also hereinafter referred to as an “AGC circuit”) 62 for adjusting the level of a voltage signal; a clamping circuit (which will be also hereinafter referred to as a “CLP circuit”) 63 for clamping the voltage signal; an AD converting circuit 64 for converting an analog signal into a digital signal; a timing control circuit 65 for generating a clock pulse to control the timing in the image pickup system 60; a timing generator/signal generator circuit (which will be also hereinafter referred to as a TG/SG circuit) 66 for generating a timing signal and driving control signal for driving and controlling the MOS sensor 61 in synchronism with the clock pulse; a DSP circuit 67 for processing the digital signal which is the output of the AD converting circuit; an encoder circuit 68 for encoding the output of the DSP circuit 67; an output circuit 69 for outputting the encoded signal; and a DA converting circuit 70 for converting the output of the output circuit 69 into an analog signal.
A picture voltage signal photoelectric-transferred by the MOS sensor 61 is level-controlled by the AGC circuit 62 to be clamped by the CLP circuit to be fed to the AD converting circuit 64. Then, the picture voltage signal is converted by the AD converting circuit 64 into a digital picture signal having one sample value of, e.g., 8 bits, to be fed to the DSP circuit 67. For example, the DSP circuit 67 comprises a color separating circuit, a clamping circuit, a gamma control circuit, a white control circuit, a black control circuit, a knee circuit, a color balancing circuit and so forth. The DSP circuit 67 carries out a required signal processing with respect to the supplied digital picture signal. Then, the signal processed by the DSP circuit 67 is fed to the encoder circuit 68. The encoder circuit 68 decodes the fed picture signal to convert it into a luminance signal and a color-difference signal. The MOS sensor 61 is timing-controlled by a timing signal and driving control signal which are fed from the TG/SG circuit 66. Thereafter, the decoded picture signal is supplied to the DA converting circuit 70 via the output circuit 69 to be converted into an analog video signal to be outputted to the outside.
In the above described image pickup system, only the image pickup region of the MOS sensor 61 has the function of converting a picture light signal into a signal charge. In other circuits than the MOS sensor 61, high density integration and speed characteristics are regarded as important. In order to improve high density integration and speed characteristics, it is required to carry out multilayering.
On the other hand, in the image pickup region of the MOS sensor 61 for handling light, there are generally formed microlenses for condensing light on the upper portion of the image pickup system. The point is whether the distance between the microlens and a photoelectric transfer region, which is formed on a semiconductor substrate and in which the photoelectric transfer is carried out, is coincident with the focal length of the microlens. That is, even if the signal processing circuit around the MOS sensor 61 is multilayered to improve high density integration and speed characteristics, the distance between the photoelectric transfer region and the microlens must be coincident with the focal length of the microlens. In addition, if an Al wiring serving as a shading layer in the photoelectric transfer region is closer to the semiconductor substrate, it prevents the incidence of irregular reflection due to shading.
In the conventional image pickup system including the MOS solid state image sensor device, image pickup characteristics are regarded as important, so that peripheral circuits in the image pickup region are not multilayered. For that reason, there is a problem in that the high density integration and accelerating of the peripheral circuits have not been realized.
When the operation of the peripheral circuits is accelerated or when circuits (including the MOS sensor) formed on the same substrate are formed of a multi layer metallization in order to facilitate design, it is difficult to condense light in the photoelectric transfer region, so that image pickup characteristics deteriorate.
Referring to FIGS. 16 and 17, these problems will be described below.
FIG. 16 is a sectional view of an image pickup system taken along line X–X′ of FIG. 15. In the image pickup system shown in FIG. 16, image pickup characteristics are regarded as important. In this image pickup system, photoelectric transfer layers 27a for converting picture light signals 40 into picture electric signals and diffusion layers 27b are formed in an image pickup region 81 of a semiconductor substrate 23 on which MOS sensors 61 are to be formed. On the top of the semiconductor substrate 23 between the photoelectric transfer layers 27a and the diffusion layers 27b, gate electrodes 25a are formed via a gate insulating film. The gate electrode 25a, the photoelectric transfer layer 27a and the diffusion layer 27b constitute the MOS transistor 61. Furthermore, the photoelectric transfer layers 27a are arranged in the image pickup region 81 in the form of a matrix. Each of the diffusion layers 27b is connected to a first Al wiring 28 via a contact provided in an interlayer dielectric film 31. Therefore, the picture electric signal converted by the photoelectric transfer layer 27a is fed to the first Al wiring 28.
In addition, shading films 29a of Al are formed in the image pickup region 81 except for the photoelectric transfer layers 27a. On the top of the interlayer dielectric film 31 directly above the photoelectric transfer layers 27a, microlenses 32 for condensing the picture light signal 40 are provided.
On the other hand, on the top of the semiconductor substrate 23 in a peripheral circuit region 82 which is element-isolated from the image pickup region 81 by element isolating regions 24 of an insulating material, MOS transistors constituting the above described circuit are formed. Each of these MOS transistors comprises a source region and drain region 26, which are formed of diffusion layers formed in the semiconductor substrate 23, and a gate electrode 25 which is formed on the semiconductor substrate 23 via the gate insulating film between the source region 26 and the drain region 26. One of the source region 26 and the drain region 26 is connected to the first Al wiring 28 via a contact provided in the interlayer dielectric film 31. The first Al wiring 28 is connected to a second Al wiring 29 via a contact provided in the interlayer dielectric film. Furthermore, the second Al wiring 29 and the shading film 29a are formed in the same layer.
In this image pickup system shown in FIG. 16, in order to allow the picture light signal 40 condensed by the microlens 32 to easily form an image on the photoelectric transfer layer 27a, a double-layer wiring structure is formed in the image pickup region 81 and the peripheral circuit region 82, and the second Al wiring 29 and the shading film 29a are thinned to decrease the distance between the photoelectric transfer layer 27a and the microlens 32 so that the distance is substantially coincident with the focal length of the microlens 32. For that reason, the high density integration and accelerating of circuits formed in the peripheral circuit region 82 are lowered.
In order to prevent the lowering of the high density integration and accelerating, there is an image pickup system shown in FIG. 17 wherein circuits formed in the peripheral circuit region 82 have a triple-layer wiring structure having first through third Al wiring layers 28, 29 and 30 and wherein the second Al wiring 29, the shading film 29a, which is formed in the same layer as the second Al wiring 29, and the third Al wiring 30 are thickened. However, in the image pickup system shown in FIG. 17, the distance between the photoelectric transfer layer 27a and the microlens 32 is longer than the focal length of the microlens 32, so that the picture light signal 40 is difficult to form an image on the photoelectric transfer layer 27a, thereby deteriorating image pickup characteristics.